EP-A 0 997 132 discloses dental glasses useful as fillers for a light curable dental composite. There is no evidence disclosed by EP-A 0 997 132 that the glasses disclosed therein are reactive and useful in a dental cement.
EP-A 0 469 573 discloses a glass ionomer cement containing a reactive glass filler and a water-insoluble heavy metal salt. The glass composition according to EP-A 0 469 573 does not contain zinc as an essential component.
U.S. Pat. No. 4,775,592 discloses a fluoroaluminosilicate glass powder for a dental glass ionomer cement, a surface of which is treated with a fluoride in a specific amount in order to improve the crushing strength and the fluidity of the cement.
Glasses form an important part of many dental restorative materials, and are used in dentistry in many different ways. A common use for glass is as an inert filler for polymerisable compositions, and in this case an inert glass is normally desirable and used. A second use for glasses in dentistry is the manufacture of crowns or inlays, and in this case it is essential that the glasses are not only inert, but also have a high surface hardness. It is also desirable for such glasses that they have a relatively low melting point so that formation of the crown or inlay is facilitated. Further examples of the use of glass in dental applications are in the so called “silicate” cements where an acid soluble glass is mixed with phosphoric acid, and “polyelectrolyte” cements where an acid soluble glass is mixed with a polyacid such as polyacrylic acid, polymaleic acid, polyvinyl phosphonate, or the like. This latter class of cements are often called glass-ionomer cements.
U.S. Pat. No. 4,814,362 discloses alkaline earth metal aluminofluorosilicate glasses suitable as ion-sources in dental glass ionomer compositions, whereby the glasses contain strontium in order to provide radioopacity. U.S. Pat. No. 5,318,929 discloses an apatite glass ceramic for a glass ionomer cement. U.S. Pat. No. 5,360,770 discloses a further glass composition for a dental glass ionomer cement. The glass compositions known from these references do not contain zinc oxide.
U.S. Pat. No. 6,355,585 discloses a glass powder for a dental glass ionomer cement compositions, wherein the bending strength and tensile strength are improved by a specific elongated shape of glass particles. The glass compositions do not contain zinc as an essential component.
A zinc containing aluminoborate glass composition for a dental glass ionomer cement is known from U.S. Pat. No. 4,336,153. Ternary aluminium-zinc-silicate glasses for the preparation of polyalkenoate glass ionomer dental cements are disclosed in Darling M.; Hill R.; Biomaterials 1994, 15(4), 299-306. However, the glass compositions disclosed therein do not contain any fluoride.
With the phosphate and polyelectrolyte cements the glass takes part in the setting reaction and is thereby partially dissolved. For these purposes the glass must not be inert, but must possess a suitable degree of acid solubility which allows partial dissolution of the glass and release of ions. Since the phosphate and polyelectrolyte cements mainly harden by crosslinking of the acids by ions released from the glass, it is obvious that the glasses therefore have to contain elements capable of being crosslinked by the acid. Monovalent ions such as Na+ and K+ are not capable of crosslinking the acids, but a wide range of multivalent ions can be used for this purpose. The composition of the glass, leading to a desired reactivity or inertness, therefore varies widely according to its intended purpose but common desirable feature for dental use is that the glass is opaque to X-rays. This enables a dentist to see a restoration with X-rays, and facilitates diagnosis of further caries, or allows the dentist to remove the restoration with minimal destruction of the remaining tooth substance. Radiopacity of glasses used for crowns and inlays is also important in case the crown or inlay is accidentally swallowed. It can therefore be seen that glasses intended for dental applications have to fulfil many exacting and varying specifications depending on their intended use.
Attempts to develop suitable glasses are described, for instance, in Journal of Dental Research June 1979 pages 1607-1619, or more recently in U.S. Pat. Nos. 4,814,362, 5,318,929, 5,360,770, and application US 2004/0079258 A1. The latter application is for an “Inert Dental Glass”, and it is claimed that this inert glass has been developed by replacing strongly basic oxides such as CaO, BaO, SrO, MgO, ZnO, Na2O, K2O, Li2O etc. with weakly basic oxides such as those in the Scandium or Lanthanide series. However MgO and ZnO are variously referred to in the application as weakly basic and suitable for replacing CaO and BaO (abstract), or as strongly basic and needing replacement (paragraph 0034) in order to obtain a suitable inert glass, and it is therefore not clear to which category MgO and ZnO are supposed to belong. In paragraph [00346] of 0079258 A1 for instance it is specifically mentioned that it was found that, by replacing or partially replacing the strongly basic ions Li+, Na+, K+, Ca+, Sr2+, Ba2+, and Zn2+ with weakly basic ions such as Sc3+, Y3+ La3+ or Ce3+ or other ions from the Lanthanide series, a glass was obtained which set significantly more slowly. Contrary to expectations from this, it is an aim of the present invention to develop a slow setting glass containing high levels of calcium, strontium and zinc ions. In particular, a glass with a high zinc content is desired. Zinc oxide has been widely used in dentistry, mainly in conjunction with phosphoric acid, polycarboxylic acids, or with eugenol. The zinc oxide forms complexes when mixed with these materials, and the resulting hard masses are useful as filling materials and cements. These zinc containing materials have proved over many years of clinical use to be particularly bland and beneficial to tooth substance, and this has been attributed to the presence of Zn2+ ions. For use with acidic formulation, the zinc oxide has to, be specially treated in order to have a sufficiently slow reaction time, and it is therefore not expected that its addition to glass will, per se, result in a slow reacting glass in a polyalkenoate formulation. A disadvantage of materials based on zinc oxide alone is that these have very poor physical properties, having a low strength, high abrasion, high water solubility, and poor aesthetics due to very high opacity. These ZnO based materials are therefore restricted to use as temporary fillings or in protected and invisible positions such as a cement under a crown. Attempts have been made to combine ZnO powder directly into a polyalkenoate cement, for instance as described in Journal of Hard Tissue Biology (2003), 12(1), 17-24. It was concluded in this study that ZnO contributes to inhibit dentine demineralization without major changes to the mechanical properties of the cement. However, ZnO is highly opaque and its incorporation in a polyalkenoate cement would reduce the aesthetics considerably. Due to the two separate setting mechanisms in this mixture, handling and setting properties are also reduced. In addition to having possible beneficial effects on the tooth and surrounding tissue, zinc is highly opaque to X-rays and helps to provide the radioopacity required in a dental filling material. In general, glasses used in polyalkenoate cements are rather too reactive and need special treatment to reduce the reactivity and provide a long enough working and setting time. Such treatment to reduce the reactivity of glasses can include heat treatment, etching with acid, coating the glass with a film, or a combination of one or more of these methods, as is described for example in JP 1991-285510. Although this is effective, it is an extra step in production which can go wrong, and also costs time and money. In addition, the surface layer of the glass is altered by etching or coating, and this surface layer is liable to later mechanical loss or abrasion during further compounding steps or transport so that the treatment becomes less effective. Calcium, as an element naturally present in teeth is also an important component of a dental glass, and its presence has been shown to encourage the formation of hydroxy apatite. Calcium can in some circumstances be replaced by strontium.